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Poly(4-vinylpyridine) (P4VP) (M v = 16 × 104 g/mol) was synthesized by radical polymerization. The molecular weight was determined using viscosity and light scattering techniques. The objective of this work is to study the interaction of P4VP (2.88 × 10−3 M) with proton H+ at 25 °C. The neutralization degrees vary between 0 and 1. This investigation was followed using UV–visible spectrophotometer and 1H NMR spectroscopy characterizations. An isobestic point was observed at α = 0.2 in 50/50 v/v water–methanol mixture. An equilibrium between the protonated and the non-protonated forms of P4VP was pronounced at α = 0.6. Constant 1H NMR chemical shifts are interpreted by stability of the structural copolymer conformation. For higher neutralization degree values, no significant changes of the structure and the conformation were observed. It was concluded that equilibrium exists between N and N+ in the P4VP/HCl system in 50 % water–methanol solution.